This invention relates to an electrochemical sensor having a working electrode, a counter electrode and a reference electrode.
When using a sensor performs an electrochemical measurement, a three-electrode method is widely employed, wherein an electrode group consisting of a working electrode, a counter electrode and a reference electrode is used. As an example of this type of sensor, a biosensor is disclosed in FIGS. 1(a)-1(b) of Japanese Patent Application under Provisional Publication No. 256812/93, wherein a working electrode, a reference electrode and a counter electrode that are molded with platinum are provided on an aminoethanesulfonic insulating substrate. As another example, an electrochemical gas sensor is disclosed in FIGS. 1(a)-1(b) and FIG. 6 of Japanese Patent Application under Provisional Publication No. 3323/94, wherein two sets of a pair of electrode group consisting of a working electrode and a counter electrode are provided for a single reference electrode.
When an electrochemical measurement by the three-electrode method is applied, it is important to confirm that each electrode functions normally before the measurement. Therefore, it is usually required to confirm the normal function of a measuring apparatus by the calibration using a calibration solution. However, in some cases an error in measured value becomes large even if the calibration is conducted. Accuracy of measurement of a sensor is dependent greatly on the characteristic of a reference electrode since a specified electric potential is applied to a working electrode on the basis of the reference electrode in a senor using the reference electrode. However, since the characteristic of the reference electrode changes due to various factors, the electric potential changes and the function as the reference electrode is sometimes greatly damaged. Although an abnormality relating to the senor as a whole apparatus may be confirmed by the calibration of the sensor before measurement, it is usually difficult to detect a delicate change of the electric potential of the reference electrode. Therefore, an accurate value of measurement may not be obtained even after the calibration described above is performed.
In addition, in the light of a convenience of a process and a small sizing of a sensor, a working electrode, a counter electrode and a reference electrode are formed on the same substrate in a normal electrochemical sensor as can be seen in the Provisional Publications described above. Therefore, the whole substrate whereon an electrode group is set has to be replaced when the reference electrode has some troubles even though the working electrode and the counter electrode are functioning normally. When the reference electrode is made of a material which tends to be damaged as compared with that used for the working electrode and the counter electrode, a restriction is imposed on a decrease in a running cost and an extension of a possible continuous measurement time depending on the life of the reference electrode and, therefore, there has still remained a room for improvement in this regard.
The circumstances mentioned above have led to this invention.
This invention relates to an electrochemical sensor based on a three-electrode method containing a reference electrode and the purpose of this invention is to detect an abnormality in the reference electrode immediately after its occurrence, recover a high reliability of measured value by removing the abnormality quickly, make it possible to conduct a continuous measurement for a long time and furthermore try to reduce a running cost.
This invention by which the problems mentioned above are solved are specified by the followings:
[1] An electrochemical sensor having a working electrode, a counter electrode, and a reference electrode, wherein a means for examining the reference electrode is provided for examining an electric potential of the reference electrode.
[2] The electrochemical sensor as describe in [1], wherein the means for examining the reference electrode comprises having an examining electrode as a standard to measure the electric potential of the reference electrode and a measuring apparatus by which a potential difference between the examining electrode and the reference electrode is measured.
[3] The electrochemical sensor as described in [1] or [2], wherein a spare reference electrode is provide for use in place of the reference electrode when the means for examining the reference electrode detects an abnormal electric potential of the reference electrode.
[4] The electrochemical sensor as described in [1] to [3], wherein an informing measures is provided to inform the time of replacing the reference electrode when the abnormal electric potential is detected by the examining measures of the reference electrode.
[5] The electrochemical sensor as described in [1] to [4], wherein a switching measures of the reference electrode is provided by which the spare reference electrode is used in place of the reference electrode when the abnormal electric potential is detected by the means for examining the reference electrode.
[6] The electrochemical sensor as described in [1] to [5], wherein an immobilized enzyme layer is formed at least on the working electrode.
[7] The electrochemical sensor as described in [6], wherein a diffusion-limiting layer containing fluoroalcohol ester of polycarboxylic acid which is formed so as to cover at least the working electrode and the reference electrode is provided on the immobilized enzyme layer.
[8] An electrochemical sensor having a working electrode, a counter electrode and a reference electrode, wherein a spare electrode is provided for use in palace of the reference electrode when a use of the reference electrode is stopped.
[9] The electrochemical sensor as described in [8], wherein
a means for switching the reference electrode is provided by which the spare reference electrode is used in place of the reference electrode when the use of the reference electrode is stopped.
[10] The electrochemical sensor as described in [8] or [9], wherein an immobilized enzyme layer is formed at least on the working electrode.
[11] The electrochemical sensor as descried in [10], wherein
a diffusion-limiting layer containing a fluoroalcohol ester of polycarboxylic acid, which is formed so as to cover at least the working electrode and the reference electrode, is provided on the immobilized enzyme layer.
An electrochemical sensor having a means for examining a reference electrode as described above can detect immediately the situation wherein the reference electrode indicates an abnormal electric potential and does not function normally. According to a prior art, the abnormality of the sensor was confirmed by a calibration, that is, a method of detecting an abnormality in the sensor was adopted as a whole sensor system.
According to this method, however, it was difficult to confirm a malfunction of the reference electrode itself. Especially, when a natural electric potential of the reference electrode is getting out of a normal value although the reference electrode is not damaged, according to the prior art a measurement is performed assuming that the reference electrode is functioning normally without detecting the occurrence of abnormality and the measurement is, therefore, performed sometimes without noticing the abnormality.
On the other hand, the electrochemical sensor having the means for examining of the reference electrode can detect a small change in the electric potential of the reference electrode and can tell the time of replacement of the reference electrode exactly and, therefore, it is possible to increase reliability on a measured value.
Various kinds of means can be adopted for examining a reference electrode and a preferable structure consists of having an examining electrode as a standard used for measuring an electric potential of the reference electrode and a measuring apparatus used for measuring a potential difference between the examining electrode and the reference electrode. There is no restriction to a material and a structure of the examining electrode and the same structure as the reference electrode, for example, may be adopted. A voltmeter, for example, may be used for the measuring apparatus.
An electrochemical sensor having a spare reference electrode as described above can reduce a running cost and furthermore conduct a continuous measurement for a long time. According to a prior art, when a reference electrode is damaged, a whole substrate on which a group of electrodes is formed has to be replaced resulting in a hindrance to a continuous measurement and an increase in the running cost. On the other hand, according to the electrochemical sensor as described above, when a damage of the reference electrode is detected, the damaged reference electrode can be switched to the spare reference electrode without replacement of the whole substrate and a measurement with a high accuracy can be continued, which results in reducing the running cost.
A spare reference electrode is used in place of a reference electrode when the reference electrode does not function normally. It is, therefore, preferable to lengthen a life of the spare electrode so that the spare electrode does not damage before the reference electrode malfunctions. However, since the same material is often used for both the reference electrode and the spare electrode for reasons of processing, it is sometimes difficult to increase the life of the spare electrode by differentiating the material to be used.
The spare reference electrode is, therefore, preferably not connected to the same power source to which a working electrode, a counter electrode and the reference electrode are connected and a voltage is also preferably not kept applied to the spare reference electrode. A photoresist may be coated on the spare electrode to lengthen the life of the spare reference electrode.
Since the sensor of this invention has an examining electrode, it is possible to detect an abnormality in the reference electrode immediately after its occurrence and increase the reliability of measured value by removing the abnormality quickly. If the sensor has a spare reference electrode, it possible to conduct a continuous measurement for a long time and to reduce the running cost.